Bone is a complex tissue composed of several cell types which are continuously undergoing a process of renewal and repair termed “bone remodeling.” The two major cell types responsible for bone remodeling are osteoclasts, which resorb bone, and osteoblasts, which form new bone. Bone remodeling has been known to be regulated by several systemic hormones (e.g., parathyroid hormone, 1,25-dihydroxy vitamin D3, sex hormones, and calcitonin) and local factors (e.g., nitric oxide, prostaglandins, growth factors, and cytokines).
Integrins are heterodimeric matrix receptors that anchor cells to substrates and transmit externally derived signals across the plasma membrane. Integrin αvβ3 is involved in the osteoclast-mediated bone resorption, both in vivo and in vitro. This heterodimer molecule recognizes the amino acid motif Arg-Gly-Asp (RGD) contained in bone matrix proteins such as osteopontin and bone sialoprotein. Integrin αvβ3 is expressed in an osteoclast and its expression is modulated by resorptive steroids and cytokines. Based on blocking experiments, αvβ3 integrin has been identified as a major functional adhesion receptor on osteoclasts. Inhibitors of integrin αvβ3 reduce the capacity of osteoclasts to bind to and resorb bone. Integrin αvβ3 plays a major role in the function of osteoclasts and inhibitors of this integrin are being considered for treating or preventing osteoporosis, osteolytic metastases, and malignancy-induced hypercalcemia.
There are many bone diseases that are related to osteolysis that is mediated by osteoclasts. Osteoporosis is the most common one that is induced when resorption and formation of bone are not coordinated and bone breakdown overrides bone building. Osteoporosis is also caused by other conditions, such as hormonal imbalance, diseases, or medications (e.g., corticosteroids or anti-epileptic agents). Bone is one of the most common sites of metastasis by human breast, prostate, lung and thyroid cancers, as well as other cancers. Osteoporosis may also result from post-menopausal estrogen deficiency. Secondary osteoporosis may be associated with rheumatoid arthritis. Bone metastasis shows a very unique step of osteoclastic bone resorption that is not seen in metastasis of other organs. It is widely accepted that osteolysis that is associated with cancer is essentially mediated by osteoclasts, which seem to be activated and may be indirectly activated through osteoblasts or directly by tumor products. In addition, hypercalcemia (increased blood-calcium concentration) is an important complication of osteolytic bone diseases. It occurs relatively frequently in patients with extensive bone destruction, and is particularly common in breast, lung, renal, ovarian and pancreatic carcinomas and in myeloma.
Disintegrins are a family of low-molecular-weight RGD-containing peptides that bind specifically to integrins αIIbβ3, α5β1 and αvβ3 expressed on platelets and other cells including vascular endothelial cells and some tumor cells. In addition to their potent antiplatelet activity, studies of disintegrins have revealed new uses in the diagnosis of cardiovascular diseases and the design of therapeutic agents in arterial thrombosis, osteoporosis and angiogenesis-related tumor growth and metastasis. Rhodostomin (Rho), a disintegrin derived from the venom of Colloselasma rhodostoma, has been found to inhibit platelet aggregation in vivo and in vitro through the blockade of platelet glycoprotein αIIbβ3. Furthermore, rhodostomin is reported to inhibit the adhesion of breast and prostate carcinoma cells to both unmineralized and mineralized bone extracellular matrices in a dose-dependent manner, without affecting the viability of tumor cells. In addition, rhodostomin inhibits the migration and invasion of breast and prostate carcinoma cells. Rhodostomin has also been shown to inhibit adipogenesis and obesity. However, because rhodostomin non-specifically binds to integrins αIIbβ3, α5β1 and αvβ3, the pharmaceutical uses of rhodostomin may cause serious side effects. For example, when applying rhodostomin in treating carcinomas, the inhibition of platelet aggregation is an undesirable side effect.
The role of αvβ3 integrin in bone diseases has been well documented. See, for example, F. Patrick Ross et al, Nothing but skin and bone, the Journal of Clinical Investigation, Vol. 116, #5, May 2006; S. B. Rodan et al, Integrin function in osteoclasts, Journal of Endocrinology (1997) 154, S47-S56; Steven L. Teitelbaum, Editorial: Osteoporosis and Integrins, the Journal of Clinical Endocrinology and Metabolism, April 2005, 90(4): 2466-2468; Steven L. Teitelbaum, Osteoclasts, integrins, and osteoporosis, Journal of Bone and Mineral Metabolism, (2000) 18: 344-349; Ichiro Nakamura et al, Involvement of αvβ3 integrins in osteoclast function, Journal of Bone and Mineral Metabolism, (2007) 25: 337-344; Le T. Duong et at, The role of integrins in osteoclast function, Journal of Bone and Mineral Metabolism, (1999) 17: 1-6; and A Teti et al, The Role of the AlphaVbeta3 Integrin in the Development of Osteolytic Bone Metastases: A Pharmacological Target for Alternative Therapy?, Calcified Tissue International (2002) 71: 293-299.
In addition to bone diseases, αvβ3 integrin plays an important role in angiogenesis and tumor growth in conditions not related to bone diseases.
Thus, it may be desirable to create polypeptides selective for αvβ3 integrin with improved stability and lasting effects. These polypeptides will be potentially suitable to treat diseases and conditions involving αvβ3 integrin, including but not limited to various bone diseases, cancer, and diseases involving angiogenesis.
It would also be advantageous to create pegylated polypeptides selective for αvβ3 integrin to prolong the duration and decrease the antigenicity of a protein drug.